In the past, as a method of production of steel excellent in weldability for the high strength steel used as steel for marine structures, the technique of controlling the cooling rate after hot rolling so as to reduce the Pcm, an indicator of weldability, has been known. Further, as a method of production of steel excellent in toughness at the HAZ (heat affected zone), for example, as described in Japanese Patent Publication (A) No. 5-171341, the technique of adding Ti to the steel material and using Ti oxides (below, TiO) as nuclei for promoting the formation of intragranular ferrite (IGF) has been known. Still further, as described in Japanese Patent Publication (B2) No. 55-26164, Japanese Patent Publication (A) No. 2001-164333, etc., the art of making Ti nitrides (below, TiN) disperse in the matrix so as to suppress the grain growth of the matrix at the time of reheating by the pinning effect and thereby secure the HAZ toughness and, as described in Japanese Patent Publication (A) No. 11-279684, the art that the Ti—Mg oxides dispersed in a matrix not only suppress grain growth at the time of reheating due to the pinning effect, but also make the ferrite finer due to the effect of promotion of formation of IGF and thereby secure the HAZ toughness are known. However, the technique of producing the above excellent HAZ toughness steel has the problems of requiring extremely complicated processes and is high in cost.
Further, in the art for making TiO or TiN finely disperse in steel to make the HAZ structure finer, the optimal values of the chemical compositions of the TiO and TiN particles and the particle sizes are also being studied. For example, Japanese Patent Publication (A) No. 2001-164333 describes that in a steel material with a ratio of Ti and N (Ti/N) of 1.0 to 6.0, including TiN particles with a particle size of 0.01 to 0.10 μm in the steel material before welding in an amount of 5×105 to 1×106/mm2 enables steel excellent in HAZ toughness to be produced.
However, to get particles to disperse as aimed at using the technique described in Japanese Patent Publication (A) No. 2001-164333, it is described that aging for 10 minutes or more at the slab cooling stage, that is, between 900 to 1300° C., is necessary. This aging at a high temperature is extremely difficult and is not preferred from the viewpoint of the heat efficiency and production capability.
On the other hand, according to Japanese Patent Publication (A) No. 7-252586, when MnS is formed in steel, the MnS forms a nuclei in the HAZ structure for promotion of formation of IGF and the crystal grain size is effectively made finer, so it is possible to secure the desired toughness. However, while there is no clear reason, since an upper limit value is set for the amount of addition of Mn in actual steel, the obtained amount of MnS is not sufficient for bringing out the effect of promotion of formation of IGF to the maximum extent.
Further, in Japanese Patent Publication (A) No. 3-264614, it is considered that in the interaction of formation of TiN and MnS, TiN functions as nuclei for precipitation of MnS. Further, an invention calling for the cooling rate at the time of solidification to be made 5.0° C./min (about 0.08° C./s) or less in the range of 1000° C. to 600° C. for the effective use of these precipitates has been proposed, but the reason for this is not quantitatively explained. For this reason, the optimal cooling rate is unclear.